Conventionally, there is a known machining method, which includes attaching a workpiece to a rotary table on an XY plane and moving a tool relative to the workpiece parallel to the X-axis while turning the workpiece to form an involute curve shaped wall part at the workpiece (for example, see Patent Literature 1). In this method described in Patent Literature 1, the tool is offset in the Y-axis direction by the amount of the radius of the base circle of the involute curve. In that state, the tool is moved toward a center part of the workpiece substantially in parallel to the X-axis to machine an inside wall surface of the wall part. After that, the tool is offset to the opposite side in the Y-axis direction by the amount of the radius of the base circle of the involute curve. In that state, the tool is moved from the center part of the workpiece in parallel to the X-axis to machine the outside wall surface of the wall part. That is, in the method described in Patent Literature 1, in the XY quadrants about the center of the workpiece as an origin, the inside wall surface is machined in one quadrant and the outside wall surface is machined in the other quadrant at the opposite side to that one quadrant across the center of the workpiece.
In this regard, in a machining unit for carrying out this type of machining method, usually when aligning the center of the tool with the center of the rotary table, off-centering of at least about several μm or so occurs between the two. Therefore, this off-centering causes the machining positions of the inside wall surface and the outside wall surface of the wall part to deviate from the design values and makes it difficult to precisely machine the thickness of the wall part between the inside wall surface and the outside wall surface.
In particular, when, like in the method described in the above-mentioned Patent Literature 1, when the inside wall surface is machined in one quadrant and the outside wall surface is machined in the other quadrant at the opposite side to that one quadrant, the deviations in machining positions of the inside wall surface and the outside wall surface are added to the thickness of the wall part and the thickness of the wall part greatly deviates from the design value.